1. Field of the Invention
The present invention relates to a method for determining electric voltage u(t) and/or electric current i(t) of an RF signal in the time domain in a calibration plane on an electrical conductor, wherein the electrical conductor has a first port at one end and the calibration plane at an opposite end, wherein, in the calibration plane, the electrical conductor is designed such that a device under test can be connected electrically with the electrical conductor in the calibration plane, wherein a component of a first RF signal which runs on the electrical conductor from the first port in the direction of the calibration plane and a component of a second RF signal which runs on the electrical conductor from the calibration plane in the direction of the first port are coupled out by means of at least one directional coupler having two outputs, wherein a time-variable first signal value v1(t) of the component of the first RF signal is measured at a first output of the directional coupler and a time-variable second signal value v2(t) of the component of the second RF signal is measured at a second output of the directional coupler, wherein for a two-port error of the directional coupler with an error matrix E:
  E  =      (                                        e            00                                                e            01                                                            e            10                                                e            11                                )  
the error terms e00, e01, e10 and e11 are determined in a first step (calibration step) as a function of a frequency f and then, in a second step (measurement step), the signal values v1(t) and v2(t) are transformed, through a first mathematical operation, into the frequency domain as wave quantities V1(f) and V2(f), wherein absolute wave quantities a1 and b1 in the frequency domain in the calibration plane are calculated from the wave quantities V1(f) and V2(f) by means of the error terms e00, e01, e10 and e11, wherein the calculated absolute wave quantities a1 and b1 are converted by means of a second mathematical operation into the electric voltage u(t) and/or the electric current i(t) of the RF signal in the time domain in the calibration plane, in accordance with the claims.
2. Description of Related Art
One of the most important measuring tasks in high frequency and microwave technology involves the measurement of reflection factors or generally—in the case of multiport devices—the measurement of scattering parameters. The network behavior of a device under test which can be described in linear terms is characterized through the scattering parameters. Frequently one is interested not only in the scattering parameters at a single measuring frequency, but in their frequency-dependency over a finitely broad measuring band. The associated measuring method is described as network analysis. Depending on the importance of the phase information in the measuring task in question, the scattering parameters can either be measured simply in terms of their value or can also be measured in complex terms; in the first case one speaks of scalar network analysis, in the second case of vectorial network analysis. Depending on the method, number of ports, and measuring frequency range, the network analyzer is a more or less complex system of test signal source and receivers which function according to the homodyne or the heterodyne principle. Because the measuring signals need to be fed to the device under test and back again through conductors and other components with unknown and sub-optimal properties, in addition to chance errors, systematic errors also occur in network analysis. These systematic errors can be minimized, within certain limits, through calibration measurements, the aim of which is to determine as many as possible of the unknown parameters of the measuring apparatus. A great number of methods and strategies exist here which differ greatly in terms of the scope of the error model used and thus in complexity and efficiency. (Uwe Siart; “Calibration of Network Analysers;” 4 Jan. 2012 (Version 1.51); http://www.siart.de/lehre/nwa.pdf)
However, scattering parameters measured with such calibration only describe linear, time-invariant devices under test completely. An extension of the scattering parameters to non-linear devices under test is represented by the X-parameters (D. Root et al: “X-parameters: The New Paradigm for Describing non-linear RF and Microwave Components.” In: tm—Technisches Messen no. 7-8, Vol. 77, 2010), which are also defined through the frequency. However, each device under test is also described through measurement of the currents and voltages or the absolute wave quantities at its ports in the time domain. Measurement in the time domain inherently includes all additional spectral components caused for example through non-linearity as well as the change over time of the device under test or its input signal. Such a time domain measurement also requires calibration. However, the aforementioned calibration methods cannot be used without modification for the measurement of absolute values since they only permit the determination of relative values (scattering parameters).
Known from WO 03/048791 A2 is a high-frequency circuit analyzer which is used to test amplifier circuits. A microwave transition analyzer (MTA) with two inputs measures two independent signal waveforms on the connected amplifier circuit which is to be tested such as, for example, incident and reflected wave, in the time domain via signal paths and ports. The measured waves are subsequently processed by means of calibration data in order to compensate the influence of the measuring system on the waves between the ports of the amplifier circuit and the input ports of the MTA. The MTA, which measures signals in the time domain with attached calibration standards, is also used in order to determine the calibration data. These signals in the time domain are transformed into the frequency domain by means of an FFT and the calibration data are then determined. Since periodic signals are only measured in the time domain, the signals are transformed into low-frequency signals prior to measurement.